Effect of supplementing veld hay with a dry meal or silage from pods of Acacia sieberiana with or without wheat bran on voluntary intake, digestibility, excretion of purine derivatives, nitrogen utilization, and weight gain in South African Merino sheep

Abstract

The study evaluated the synergism between wheat bran and tanniferous feeds (dry meal or silage from pods of Acacia sieberiana) on intake and digestibility of various nutrients as well as weight gain in sheep fed a basal diet of veld hay ( Cynodon dactylon). Dietary treatments comprised veld hay given ad libitum and supplemented with: 270 g of wheat bran (WB) alone (T 1), 203 g of WB plus 83 g of silage (T 2), 135 g of WB plus 165 g of silage (T 3), 330 g silage without WB (T 4) or 306 g of dry pods without WB (T 5). The silage was prepared by mixing ground pods with water in a ratio of 40:60 (weight of pod:weight of water) and ensiling in 200-l drums for 35–45 days. Thirty young South African Merino sheep (15 males and 15 females), averaging 24 kg liveweight and 8 months of age, were blocked by sex and weight and within block, randomly distributed to the dietary treatments. A growth study was followed by a metabolism phase. All animals were slaughtered at the end of the trial to evaluate the weight of the dressed carcasses and some body organs. Faecal samples were examined at the beginning and at the end of the trial in order to evaluate the effect of tannins on endoparasites. Dry matter intakes for treatments 1, 2, 3, 4 and 5 were 1017, 1087, 1169, 1238 and 1214 g, respectively. Average liveweight gains (ADG) were 97, 114,114, 132 and 123 g in the same order. The addition of silage increased ( P<0.01) hay and DM intake but had no effect ( P>0.05) on liveweight gain and feed conversion efficiency. Increasing the levels of silage in the diet linearly ( P<0.01) increased ADG. Sex did not influence intake or ADG. The digestibilities of DM, OM and detergent fibres as well as excretion of purine derivatives were similar ( P>0.05) among treatments but the WB diet promoted higher N digestibility. The pod and silage diets increased ( P<0.01) faecal N and lowered ( P<0.05) urinary N excretion by sheep. N retention was similar among treatments. Carcass, liver and kidney weights were similar ( P>0.05) among treatments and the organs had no lesions. The results show that combining WB and silage did not show significant synergism but condensed tannins present in the silage and pod supplements produced beneficial effects probably by forming tannin–protein complexes which by-passed the rumen and were digested at the lower segments of the gastro-intestinal tract. The results of the study have shown that the pods of Acacia sieberiana can constitute an important source of nitrogen to livestock and the liveweight gains are indications that, if legume pods are well managed, they can reduce weight loss in animals grazing low quality rangelands and enhance production.